In recent years, in order to improve the efficiency of steam turbines, the length of the blade used for the steam turbines has also been increased. For example, when a long blade material having a length of over about 1500 mm is manufactured, a method is mainly used in which a material is sandwiched between an upper mold and a lower mold and is then formed into a blade material by a large press forging machine.
However, in the above-described method, since large working force of 10,000 tons or more is needed, an investment in equipment including the forging machine is very large, and also the manufacturing cost of the molds is very high.
On the other hand, methods for manufacturing a long blade material by using a forging machine having a relatively small capacity have also been tried. These methods are technically divided into two main categories. The first category includes methods, as represented by, for example, JP-A-62-192223 (Patent Literature 1) filed by the present applicants, in which methods the area to be forged is divided into a plurality of areas and then a blade material is formed by forging the divided areas. The second category includes methods, as represented by, for example, JP-A-63-241118 (Patent Literature 2), in which methods a semi-finished product is obtained by forging a material while maintaining a horizontal state of the material, and then a blade material is formed by twisting the semi-finished product.